This invention relates to acoustical flowmeter systems and is particularly directed to an improvement in the acoustical flowmeters of the type described and claimed in the U.S. Pat. No. 4,003,252 entitled "Acoustical Wave Flowmeter" by E. J. DeWath which issued Jan. 18, 1977 and the flowmeter system of the type described and claimed in the U.S. Pat. No. 4,164,865 entitled "Acoustical Wave Flowmeter" by L. G. Hall and R. S. Loveland which issued Aug. 21, 1979.
The invention of DeWath was directed to a flow meter having an unobstructed tubular wall thereby eliminating all impediments to the flow path of the fluid and eliminating all cavities in which debris might collect. The advantages of such a configuration is fully set forth in the DeWath Patent. To measure flow of a selected fluid in the DeWath flowmeter, however, required a calibration for that particular fluid and required a recalibration if the flow of a different fluid was to be measured since the flowmeter was not responsive to changes in fluid species or densities.
The Hall and Loveland invention improved the DeWath flowmeter by providing a flowmeter that measured flow accurately regardless of changes in fluid composition or temperature and by providing a flowmeter with a means for determining a change in velocity of sound of the fluid being measured.
In order to accomplish this, the Hall and Loveland acoustical wave flowmeter system had two spaced apart crystal transducers in the wall of the flowmeter conduit (sometimes called a cavity) to produce ultrasonic acoustic compressions at selected frequencies in the fluid within the cavity. The transducers were alternately switched into a transmit and a receive mode to generate upstream and downstream transmitted and received signals with an automatic means to adjust the transmitted frequencies to compensate for changes in velocity of the acoustic compressions in the fluid caused by changes in fluid composition and temperature. The electronic circuitry involved in the Hall and Loveland flowmeter system includes means for measuring and storing signals representing the phase difference between the transmitting transducer signal producing the acoustic compressions and the signal produced by the receiving transducer during each of two successive transmit/receive cycles. Circuit means were provided to determine the difference between the signals representing the two successive phase differences wherein the sign of the difference corresponds to the direction of the fluid flow and the magnitude of the difference corresponds to the rate of fluid flow through the flowmeter. Circuit means were also provided to add the two successive phase difference signals together to obtain a signal proportional to the velocity of sound in the fluid moving through the flowmeter. This latter signal indicated the change in composition of the fluid flowing through the meter.
In the DeWath flowmeter and the Hall and Loveland flowmeter system, the crystal transducers producing the energy in the fluid in the cavity, transmitted alternately with a period in between such transmissions called "dead" time. This dead time was an interval after the time one transmission ceased and before the other transmission started. Thus, one transducer would transmit and then, after a period of time, the other transducer would transmit, then, after a period of time, the first transducer would transmit again, and so on. It was thought that the acoustical energy in the fluid in the cavity must be allowed to decay to obtain the maximum results, hence the need for the dead times and, in an effort to reduce noise and offset drift, experiments in transmission times and dead times were made. The transmission times were changed and the dead times were changed slightly but to no avail. No matter what periods of time for the transmission and dead times were selected, the system did not improve.
This experimentation turned out to be the wrong approach, because it was found quite unexpectedly, that it was not necessary for the energy in cavity to decay and that, as a matter of fact, quite the opposite was true. If the dead times were eliminated altogether and if the second transmission followed immediately after the first transmission, the energy of the second transmission "swamped out" the energy of the first transmission. This discovery allowed the transmission times to be longer within the same transmit receive cycle as before, thus permitting the transmitting energy to equilibrate. Allowing the transmitted energy to equilibrate permitted more accurate measurements of the waves in the cavity. This resulted in an improvement in the accuracy of the measured zero and span (system gain).
More specifically, with the first transmission remaining ON a longer time and with the second transmission from the other transducer occuring immediately thereafter, and remaining ON for a longer time, energy in the fluid in the cavity has more time to equilibrate and thus more accurate measurements can be made. For example, in the Hall and Loveland patent, the transmission times for the transducers, shown as X and Y pulses, were of 2.5 milliseconds in length with an interval of dead time of 2.5 milliseconds in between. When one transducer was transmitting, a phase difference measurement was made after the transmission was half way through its time period, that is, after 1.25 milliseconds had elapsed. This was to allow for the energy to equilibrate in the flow cavity but, since this equilibration time was only 1.25 milliseconds long, inaccuracies in the measurement occurred. This was particularly critical when the flowmeter was subjected to different fluid constituents which would cause the transmitted energy to change. This inaccurancy was also apparent when zero measurements were attempted, ie, measurements of the energy in the cavity under conditions of no fluid flow. Without sufficient time to equilibrate, drift in zero measurements occurred.
With the increase in transmission time, the energy is now allowed 3.75 milliseconds to equillibrate before the phase measurement period of 1.25 milliseconds; all within the same time frame.
Accordingly, it is a primary object of this invention to provide a flowmeter system for measuring fluid flow and density with improved performance having less noise, increased span stability and with the minimization of drift in zero flow measurements.
Still another and more specific object of this invention is to provide a flowmeter system with increased transmission times to give the energy in the cavity produced by such transmissions more time to equilibrate, by eliminating dead periods between transmissions thus allowing the following transmission to swamp out the energy in the cavity produced by the prior transmission.
Another object of this invention is to provide a method of measuring fluid flow by providing a flowmeter with alternate first and second transmissions of energy into the fluid into the cavity with the second transmission and immediately following the first transmission thus swamping the energy in the fluid due to the first transmission.